1. Field of the Invention
This invention relates to a wrist watch with an alarm, which emits audible time sound produced by a vibrating plate to be vibrated by applying AC voltage to a piezoelectric element forming vibratory plate.
2. Prior Art
In conventional wrist watches of this type, alarm sound is produced inside a watch case where a diaphragm having a piezoelectric element is mounted. Such wrist watches are hermetically constructed for water-proofing or dust-proofing. Consequently, the alarm sound produced inside the watch case is barely audible, in most cases.
The wrist watch, as shown in FIG. 1 for example, discloses a solution to this problem. In FIG. 1 a reference numeral 1 represents a watch case on which a threaded back cover 2 is mounted with an O-ring 3 given pressure therewith. The back cover 2 has some holes 2a for passing sound, said holes being made on the bottom face of the back cover, and a disk diaphragm 4 fixed on the inner stepped portion. The diaphragm 4 consists of a base electrode 4a fixed at the periphery on the back cover 2, a piezoeletric element 4b attached to the inner side of the base electrode 4a and an electrode 4c formed on the inner side of the piezoelectric element 4b. The electrode 4c is connected to a circuit board 6 through a resilient lead 5. The circuit board 6 is mounted on a movement 7 which is housed inside the watch case 1.
In such a wrist watch as shown in FIG. 1, the piezoelectric element 4b is vibrated when voltage is applied between the base electrode 4a and the electrode 4c, and a vibration or alarm sound is produced at the center of the diaphragm 4. Said alarm sound is transmitted to the outside through the holes 2a.
In the wrist watch shown in FIG. 1, it is certain that the alarm sound produced therein can be transmitted to the outside with less resistance. However, the wrist watch cannot be kept hermetic or water-tight because of the holes 2a for sound passage, and neither water nor dust can be prevented from entering the watch. Furthermore, such holes are undesirable from the viewpoint of good appearance.
Thus, the wrist watch shown in FIG. 2 is designed to overcome the above disadvantages. This wrist watch includes a piezoelectric element 14b attached directly to a back cover 12 as a base, an electrode 14c formed on the other side of the piezoelectric element 14b, said electrode 14c being connected to a circuit board 16 mounted on a movement 17 through a lead 15.
In the wrist watch shown in FIG. 2, the back cover 12 itself is vibrated so as to produce alarm sound. Consequently, said alarm sound can be transmitted to the outside without any obstacle. It is, however, difficult to generate a sound of a high amplitude due to the back cover 1, which is commonly made of stainless steel having high rigidity, and therefore, sufficient alarm sound cannot be produced.
Furthermore, in prior art wrist watches vibrations may occur at the center of a diaphragm, and a high driving voltage is required for producing sound of a high amplitude. Such a low driving voltage as employed in these wrist watches would not be good enough, resulting in poor acoustic conversion efficiency. In addition, a plate-like diaphragm can be vibrated with small amplitude due to its high spring constant in vibrations, and has a high characteristic frequency, which is disadvantageous. A thinner diaphragm has been suggested for eliminating this drawback; however, this may result in cracking the piezoelectric element fixed on the base electrode and reducing the strength of the diaphragm itself unfavorably.